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7d2d8c6747
QEMU had no host-side interface at all: every adapter was a local UDP tunnel
into uBridge (-netdev socket plus two UDP ports per adapter), so a link could
only ever be a userspace relay. Each adapter now owns a persistent TAP
(gq{node_id[:8]}e{adapter}p{port}), created by uBridge before QEMU is launched
and handed to this user, and the netdev is that TAP -- the anchor role Docker's
veth host end plays. Links attach to it:
* kernel link (NIOBridge): brctl create/up/addif enslaves the TAP into the
per-link bridge and capture, markers and tc filters are installed on it,
verbatim through the shared KernelDatapathMixin -- a TAP and a veth host end
are interchangeable there (an egress qdisc on either impairs traffic entering
the node, and PACKET_OUTGOING on either is the node receiving);
* relay link: bridge add_nio_ethernet plus add_nio_udp -- AF_PACKET, never the
TAP NIO, which would take the fd QEMU itself holds.
The netdev type is fixed at launch, so this is all-or-nothing per VM: on the
TAP datapath every link type attaches to (or detaches from) a running VM,
including switching a link between the two datapaths. Stop un-persists the
TAPs (QEMU's open fd keeps each device alive until it exits) and removes the
per-link bridges; start sweeps leftovers first. A uBridge without the tap
module keeps the previous socket-netdev datapath, so an unupgraded install is
unaffected -- such a node simply cannot carry kernel links.